mirror of
https://github.com/matrix-construct/construct
synced 2024-11-16 15:00:51 +01:00
95 lines
4.5 KiB
C++
95 lines
4.5 KiB
C++
// Matrix Construct
|
|
//
|
|
// Copyright (C) Matrix Construct Developers, Authors & Contributors
|
|
// Copyright (C) 2016-2019 Jason Volk <jason@zemos.net>
|
|
//
|
|
// Permission to use, copy, modify, and/or distribute this software for any
|
|
// purpose with or without fee is hereby granted, provided that the above
|
|
// copyright notice and this permission notice is present in all copies. The
|
|
// full license for this software is available in the LICENSE file.
|
|
|
|
#if __has_include("db/write_thread.h")
|
|
#define ROCKSDB_PLATFORM_POSIX
|
|
#include "db/write_thread.h"
|
|
#endif
|
|
|
|
#ifndef STORAGE_ROCKSDB_INCLUDE_DB_H_
|
|
#warning "The RocksDB source code is not available. Cannot interpose bugfixes for db/write_thread.h."
|
|
#endif
|
|
|
|
///////////////////////////////////////////////////////////////////////////////
|
|
//
|
|
// This unit exists to mitigate an instance of a bug in RocksDB documented by
|
|
// https://github.com/facebook/rocksdb/issues/4654. In summary, some RocksDB
|
|
// code makes direct use of std::mutex and std::condition_variable unlike the
|
|
// rest of RocksDB code which uses the rocksdb::port and rocksdb::Env wrapper
|
|
// interfaces. We have adapted the latter to work with ircd::ctx userspace
|
|
// threading (see: db_port.cc and db_env.cc), but the former is a direct
|
|
// interface to kernel pthreads which are incompatible in this context.
|
|
//
|
|
// Our mitigation is made possible by dynamic linking. It is a legitimate use
|
|
// of runtime interposition as stated in official documentation for this exact
|
|
// purpose: overriding buggy functions in library dependencies.
|
|
//
|
|
// This unit overrides a class member function in rocksdb::WriteThread which
|
|
// originally made use of pthread primitives to handle two threads contending
|
|
// for write access in RocksDB's single-writer design. This function is entered
|
|
// by additional threads after a first thread is an established "write leader."
|
|
// These additional threads wait until a state bitmask satisfies them so they
|
|
// can continue. This waiting is accomplished with an std::condition_variable
|
|
// which tells the kernel to stop the thread until satisfied. Since we are not
|
|
// using kernel-driven threads, this is a deadlock.
|
|
//
|
|
// Our solution is not ideal, but operates effectively and is rarely invoked
|
|
// given our prior mitigations to eliminate write contention inside RocksDB.
|
|
// This code may be hit during simultaneous flush and compaction jobs, which is
|
|
// a rare event with our configuration. When hit, our yield loop tends to have
|
|
// one iteration. Our solution using a coarse yield, though, is not ideal: the
|
|
// waiting ircd::ctx is not actually being notified of a state mask change
|
|
// because the write-leader is notifying an std::condition_variable which leads
|
|
// to nowhere. Our coarse yield simply puts the waiting ircd::ctx at the back
|
|
// of the ircd::ios queue (similar to a sched_yield(2)) and hopes that the
|
|
// write-leader has finished its task and updated the state mask before the
|
|
// waiter reaches the front of the queue to check. In the worst-case, the
|
|
// waiter will requeue itself to check the state mask too often. We can make
|
|
// further improvements here by also overriding the notification mechanism
|
|
// as well, but we do not feel it's required at this time, as noted.
|
|
//
|
|
// Unfortunately, we are overriding a C++ class member function to achieve this.
|
|
// Class member functions require the correct class definition. This means we
|
|
// require a copy of the RocksDB source code to be accessible at build-time. If
|
|
// no copy of the RocksDB source is found when this unit is built, it is not an
|
|
// error, but the mitigation will not be made and the result will be vulnerable
|
|
// to this deadlock.
|
|
|
|
#ifdef STORAGE_ROCKSDB_INCLUDE_DB_H_
|
|
uint8_t
|
|
rocksdb::WriteThread::BlockingAwaitState(Writer *const w,
|
|
uint8_t goal_mask)
|
|
{
|
|
// Create the class member mutex and cv where it's expected by
|
|
// rocksdb callers
|
|
w->CreateMutex();
|
|
|
|
auto state(w->state.load(std::memory_order_acquire));
|
|
assert(state != STATE_LOCKED_WAITING);
|
|
if((state & goal_mask) == 0 && w->state.compare_exchange_strong(state, STATE_LOCKED_WAITING))
|
|
{
|
|
size_t yields(0);
|
|
while((state = w->state.load(std::memory_order_relaxed)) == STATE_LOCKED_WAITING)
|
|
{
|
|
ircd::ctx::yield();
|
|
++yields;
|
|
}
|
|
|
|
// Since we're using a coarse ctx::yield() it's theoretically possible
|
|
// that our loop can spin out of control. That is highly unlikely,
|
|
// and there is usually not even more than one iteration. Nevertheless
|
|
// we assert to be sure this is working within reason.
|
|
assert(yields < 32);
|
|
}
|
|
|
|
assert((state & goal_mask) != 0);
|
|
return state;
|
|
}
|
|
#endif STORAGE_ROCKSDB_INCLUDE_DB_H_
|